| The primary requirement for measuring the noise
emitted by products is usually some form of
frequency information. This can be as simple as the
octave
bands of noise but may require more
detail such as that given by
third
octave band analysis. For very detailed
analysis of finely spaced tonal noise problems then narrow band
analysis in the form of
Fast
Fourier Transform, or FFT, data may be
needed. Suitable meters are the
CEL-440.B or the
CEL-450.B octave band analyzer
or the
CEL-553.C third octave band analyzer. The CEL-920 FFT based pc software system
can be used for detailed frequency analysis. |
narrow band spectrum from CEL-920 FFT analyzer |
| Good accuracy is often mandated by the
product noise standards published by the international
authorities and this will involve the use of Precision or
type 1 accuracy sound level meters. These are more
accurate than the
type 2 general purpose meters which are
satisfactory for the assessment of workplace noise. The
difference between type 1 and type 2 is usually small for
mid frequency noises but can be significant when the
noise contains strong low or high frequency components.
Laboratory tests show type 1 meters are accurate to +/-
0.7 dB at reference conditions (1 kHz tone, standard
temperature and pressure, microphone pointing directly at
source) but can typically be about 1.5 dB in real
situations. Similar figures for type 2 meters show real
situation tolerances of as much as +/- 2.3 dB. (Figures
from
ANSI S1.4) It must be realized that a change of +3
dB represents a doubling of the sound energy due to the
logarithmic nature of the
decibel. |
ANSI type 1 & 2 accuracy with frequency |
| Often the
sound power
level of a machine or piece of
equipment will be required. This may have to be measured in a
special room where the external background level can be reduced
sufficiently to get good quality results. These rooms are
sometimes referred to as
anechoic
rooms and are covered with
absorbent
material such as foam on the inside walls to cut down on
reflections in the room. Hard reflective rooms called
reverberation
chambers are also used to measure the
sound power of machines. Some standards call for an item to be
measured over a hard reflecting surface and taking a machine
outside to measure its noise output on a car park away from
buildings can often be used as a measurement technique. If a
machine has to be measured in a noisy workshop because there is
no special room available then the
background noise level should be at least 10 dB below the level of
the item under test in order to get valid measurements. This may
have to be carried out when the normal shift work has finished or
at weekends when it is sufficiently quiet to obtain good
measurements. |
noise source under investigation |
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